1. Field of the Invention
The invention relates to a method for manufacturing a composite positive electrode active material that is useful, for instance, as a positive electrode active material in lithium secondary batteries. More particularly, the invention relates to a method for manufacturing a composite positive electrode active material that allows obtaining a composite positive electrode active material that has excellent output characteristics, by forming a composite in which aggregation of carbon nanotubes is suppressed and dispersibility between a positive electrode active material and carbon nanotubes is enhanced.
2. Description of the Related Art
Among various types of batteries, lithium secondary batteries are widely used as power sources in small portable electronic devices, portable information terminals and the like thanks to the advantages of lithium secondary batteries in terms of light weight, high output and high energy density. Lithium secondary batteries have become thus one mainstay of the information society. Lithium secondary batteries have also attracted attention as power sources in electric and hybrid vehicles, which demand larger and safer batteries having yet higher energy densities.
To store lithium in the positive electrode active material of positive electrodes of lithium secondary batteries, electrons must be transported to the positive electrode active material. The positive electrode active material, however, has low election conductivity, and hence a conductive substance such as carbon or the like is added to increase electron conductivity. Conventionally, the substance used as such a conductive substance has been mainly carbon black. There are hints that the composite structure of positive electrode active materials and conductive substances exert an influence on battery performance, namely high-rate charge and discharge characteristics. The use of conductive substances in the form of carbon nanotubes (CNT), which have a finer structure, is thus being explored.
For instance, Japanese Patent Application Publication No. 2008-34376 (JP-A-2008-34376) discloses a composite positive electrode material obtained by forcibly dispersing carbon nanotubes and a positive electrode active material in a solvent, followed by drying and solidification to yield a composition into which a conductive substance having a predetermined aspect ratio is then mixed. Also, Japanese Patent Application Publication No. 2008-130526 (JP-A-2008-130526) and Japanese Patent Application Publication No. 2005-123107 (JP-A-2005-123107) disclose methods for manufacturing a carbon-containing active material by mixing, for instance, a starting material of an active material having an olivine structure, with a carbon material and so forth, and by subjecting the obtained mixture to a heating treatment in a hydrothermal synthesis method.
In JP-A-2008-34376, the conductive substance having is predetermined aspect ratio is intercalated between composite particles of the positive electrode active material to which the carbon nanotubes are adhered, as a result of which there is presumably achieved a structure having sufficient conduction paths, and into which an electrolyte solution seeps smoothly, so that the structure exhibits excellent Li ion diffusion. However, carbon nanotubes have high self-aggregation, and aggregate as individual carbon nanotubes. Therefore it is not possible to obtain a positive electrode active material in which carbon nanotubes are homogeneously dispersed. Japanese Patent Application Publication No. 2009-81072 (JP-A-2009-81072) discloses a method for manufacturing an electrode body in which a positive electrode active material is produced in accordance with a hydrothermal synthesis method, and thereafter, a conductive agent or the like is added to the positive electrode active material, without drying, end the obtained paste is coated onto to collector. Aggregation of the positive electrode active material is suppressed as a result. However, JP-A-2009-81072 discloses nothing concerning aggregation of a conductive substance in a composite of a positive electrode active material and a conductive substance.